Problem: In history class, the girl to boy ratio is $9$ to $8$. If there are a total of $68$ students, how many girls are there?
A ratio of $9$ girls to $8$ boys means that a set of $17$ students will have $9$ girls and $8$ boys. A class of $68$ students has $4$ sets of $17$ students. Because we know that there are $9$ girls in each set of $17$ students, the class must have $4$ groups of $9$ girls each. There is a total of $36$ girls in history class.